Switch from _Noreturn to __attribute__((noreturn))

Parts of the Linux kernel build system struggle with _Noreturn.  This
results in the following warnings when building on RHEL 8.5, and likely
other environments.  Switch to using the __attribute__((noreturn)).

  warning: objtool: dbuf_free_range()+0x2b8:
    return with modified stack frame
  warning: objtool: dbuf_free_range()+0x0:
    stack state mismatch: cfa1=7+40 cfa2=7+8
  ...
  WARNING: EXPORT symbol "arc_buf_size" [zfs.ko] version generation
    failed, symbol will not be versioned.
  WARNING: EXPORT symbol "spa_open" [zfs.ko] version generation
    failed, symbol will not be versioned.
  ...

Additionally, __thread_exit() has been renamed spl_thread_exit() and
made a static inline function.  This was needed because the kernel
will generate a warning for symbols which are __attribute__((noreturn))
and then exported with EXPORT_SYMBOL.

While we could continue to use _Noreturn in user space I've also
switched it to __attribute__((noreturn)) purely for consistency
throughout the code base.

Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13238

module/zfs/vdev_removal.c

@@ -140,7 +140,7 @@ int zfs_removal_suspend_progress = 0;
 
 #define	VDEV_REMOVAL_ZAP_OBJS	"lzap"
 
-static _Noreturn void spa_vdev_remove_thread(void *arg);
+static __attribute__((noreturn)) void spa_vdev_remove_thread(void *arg);
 static int spa_vdev_remove_cancel_impl(spa_t *spa);
 
 static void
@@ -1589,7 +1589,7 @@ spa_remove_max_segment(spa_t *spa)
  * TXG have completed (see spa_txg_zio) and writes the new mappings to disk
  * (see vdev_mapping_sync()).
  */
-static _Noreturn void
+static __attribute__((noreturn)) void
 spa_vdev_remove_thread(void *arg)
 {
 	spa_t *spa = arg;